from functools import reduce
from typing import List
import json

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        ret = 0
        for n in nums:
            ret ^= n
        print(reduce(lambda x, y: x ^ y, nums))
        return ret
    

def main():
    import sys
    import io
    def readlines():
        for line in io.TextIOWrapper(sys.stdin.buffer, encoding='utf-8'):
            yield line.strip('\n')
    lines = readlines()
    while True:
        try:
            line = next(lines)
            nums = json.loads(line)
            print(nums)
        except StopIteration:
            break
            

if __name__ == '__main__':
    nums0 = json.loads(input())
    print(Solution().singleNumber(nums0))
    